A planetary nebula is an emission nebula consisting of a glowing shell of gas and plasma formed by certain types of stars when they
die.
- Wikipedia - Planetary NebulaA planetary nebula is created when a star blows off its outer layers after it has run out of fuel to burn. These outer layers of gas expand
into space, forming a nebula which is often the shape of a ring or bubble. About 200 years ago, William Herschel called these spherical clouds planetary nebulae because
they were round like the planets. At the center of a planetary nebula the glowing, left-over central part of the star from which it came can usually still be
seen.- Ask an
Astronomer, Cool Cosmos at the Infrared Processing and Analysis Center & the SIRTF Science CenterThe planetary nebula is part of a star's stellar life cycle, it's own contribution to the cosmic art gallery if you will. Having a mass up to
eight to twelve times greater than our own sun and now out of fuel to burn the star ejects it's outer layers, shedding them to the stellar winds it creates, leaving behind a
brillantly glowing central core called a white dwarf. A shell or 'bubble' of plasma & gases form out of the star's shedded material which is then ionized by it's own
ultraviolet radition, causing the shell to glow.- K.Pinkela

NEBULA PAGE GOAL:"To Share an awe inspiring beauty who's life is only seconds long when measured against the universe's scale of
time."Ten to fifteen-thousand years or so old may sound like a mighty long time to live however, when compared with the
age of a star, millions to billions of years old, your still just an infant. Taking a mid-range example of a star's age, say 6 billion years, a single nebula could live out
600,000 life times or more. But short-lived or not the Nebula is still able to enjoy a fairly active stellar existence while it's benefactor, the red giant star, is now fighting
for it's own. Having consumed most of the hydrogen fuel that feed it's own fusion procecss the star begins to cool down and change:

"...stars convert hydrogen to helium to produce light (and other radiation). As time progresses, the heavier helium sinks to the center
of the star, with a shell of hydrogen around this helium center core. The hydrogen is depleted so it no longer generates enough energy and pressure to support the outer
layers of the star. As the star collapses, the pressure and temperature rise until it is high enough for helium to fuse into carbon, i.e. helium burning begins. To radiate
the energy produced by the helium burning, the star expands into a Red Giant."- Jeff Silvis, Ask an
Astrophysicist

Thereafter, the expanding red giant sheds or casts off with stellar-like winds it outer layers, leaving behind an inert
core consisting mostly of carbon and oxygen and known as a white dwarf star. And it is the white dwarf who radiates out and over the dust, gases and plasma of
it's own remains.

Planetary and Diffused Nebulae - These are the two
general catagories we use when classifying nebulae. Diffused Nebula, the larger of the two, have enough dust and gas to produce the equivelant of 100,000 stars the size
of our sun. The three types of diffused nebulae are Emission Nebulae, Reflection Nebulae and Supernova Remnants.

Emission Nebula - This type of nebula is created when
energetic ultraviolet light from a hot star shines on a cloud of hydrogen gas, stripping away electrons from the atoms (ionization). With large amounts of
electrons recombining in the cloud the emitted light ("emission"), as well as the nebula's cloud, is seen in the color red.

HII Ionized Region - A region of hot gas surrounding a
young star or stars that is mostly ionized. The energetic light from these young stars ionizes the existing gas. This region typically appears red as it glows with the
photons emitted when electrons recombine with hydrogen protons.

Reflection Nebula - A type of nebula that shines by
reflected light. Bright stars near reflection nebulae emit light into the region that is reflected by the large amount of dust there. The size of the dust
grains causes blue light to be reflected more efficiently than red light, so these reflection nebulae frequently appear blue in color. Plural of nebula: nebulae.

Supernova Remnants - "A supernova remnant (SNR) is the
structure resulting from the gigantic explosion of a star in a supernova. The supernova remnant is bounded by an expanding shock wave, and consists of ejected material
expanding from the explosion, and the interstellar material it sweeps up and shocks along the way." - Wikipedia